Add simplified critical path scheduler to improve build times

src/build.cc

@@ -89,6 +89,7 @@ void Plan::Reset() {
 }
 
 bool Plan::AddTarget(const Node* target, string* err) {
+  targets_.push_back(target);
   return AddSubTarget(target, NULL, err, NULL);
 }
 
@@ -123,8 +124,6 @@ bool Plan::AddSubTarget(const Node* node, const Node* dependent, string* err,
   if (node->dirty() && want == kWantNothing) {
     want = kWantToStart;
     EdgeWanted(edge);
-    if (!dyndep_walk && edge->AllInputsReady())
-      ScheduleWork(want_ins.first);
   }
 
   if (dyndep_walk)
@@ -151,10 +150,10 @@ void Plan::EdgeWanted(const Edge* edge) {
 Edge* Plan::FindWork() {
   if (ready_.empty())
     return NULL;
-  EdgeSet::iterator e = ready_.begin();
-  Edge* edge = *e;
-  ready_.erase(e);
-  return edge;
+
+  Edge* work = ready_.top();
+  ready_.pop();
+  return work;
 }
 
 void Plan::ScheduleWork(map<Edge*, Want>::iterator want_e) {
@@ -175,7 +174,7 @@ void Plan::ScheduleWork(map<Edge*, Want>::iterator want_e) {
     pool->RetrieveReadyEdges(&ready_);
   } else {
     pool->EdgeScheduled(*edge);
-    ready_.insert(edge);
+    ready_.push(edge);
   }
 }
 
@@ -437,6 +436,121 @@ void Plan::UnmarkDependents(const Node* node, set<Node*>* dependents) {
   }
 }
 
+namespace {
+
+template <typename T>
+struct SeenBefore {
+  std::set<const T*>* seen_;
+
+  SeenBefore(std::set<const T*>* seen) : seen_(seen) {}
+
+  bool operator() (const T* item) {
+    // Return true if the item has been seen before
+    return !seen_->insert(item).second;
+  }
+};
+
+// Heuristic for edge priority weighting.
+// Phony edges are free (0 cost), all other edges are weighted equally.
+int64_t EdgeWeightHeuristic(Edge *edge) {
+  return edge->is_phony() ? 0 : 1;
+}
+
+}  // namespace
+
+void Plan::ComputeCriticalPath() {
+  METRIC_RECORD("ComputeCriticalPath");
+  // Remove duplicate targets
+  {
+    std::set<const Node*> seen;
+    SeenBefore<Node> seen_before(&seen);
+    targets_.erase(std::remove_if(targets_.begin(), targets_.end(), seen_before),
+                   targets_.end());
+  }
+
+  // Use backflow algorithm to compute the critical path for all
+  // nodes, starting from the destination nodes.
+  // XXX: ignores pools
+  std::queue<Edge*> work_queue;        // Queue, for breadth-first traversal
+  // The set of edges currently in work_queue, to avoid duplicates.
+  std::set<const Edge*> active_edges;
+  SeenBefore<Edge> seen_edge(&active_edges);
+
+  for (size_t i = 0; i < targets_.size(); ++i) {
+    const Node* target = targets_[i];
+    if (Edge* in = target->in_edge()) {
+      int64_t edge_weight = EdgeWeightHeuristic(in);
+      in->set_critical_path_weight(
+          std::max<int64_t>(edge_weight, in->critical_path_weight()));
+      if (!seen_edge(in)) {
+        work_queue.push(in);
+      }
+    }
+  }
+
+  while (!work_queue.empty()) {
+    Edge* e = work_queue.front();
+    work_queue.pop();
+    // If the critical path of any dependent edges is updated, this
+    // edge may need to be processed again. So re-allow insertion.
+    active_edges.erase(e);
+
+    for (std::vector<Node*>::iterator it = e->inputs_.begin(),
+                                      end = e->inputs_.end();
+         it != end; ++it) {
+      Edge* in = (*it)->in_edge();
+      if (!in) {
+        continue;
+      }
+      // Only process edge if this node offers a higher weighted path
+      const int64_t edge_weight = EdgeWeightHeuristic(in);
+      const int64_t proposed_weight = e->critical_path_weight() + edge_weight;
+      if (proposed_weight > in->critical_path_weight()) {
+        in->set_critical_path_weight(proposed_weight);
+        if (!seen_edge(in)) {
+          work_queue.push(in);
+        }
+      }
+    }
+  }
+}
+
+void Plan::ScheduleInitialEdges() {
+  // Add ready edges to queue.
+  assert(ready_.empty());
+  std::set<Pool*> pools;
+
+  for (std::map<Edge*, Plan::Want>::iterator it = want_.begin(),
+           end = want_.end(); it != end; ++it) {
+    Edge* edge = it->first;
+    Plan::Want want = it->second;
+    if (!(want == kWantToStart && edge->AllInputsReady())) {
+      continue;
+    }
+
+    Pool* pool = edge->pool();
+    if (pool->ShouldDelayEdge()) {
+      pool->DelayEdge(edge);
+      pools.insert(pool);
+    } else {
+      ScheduleWork(it);
+    }
+  }
+
+  // Call RetrieveReadyEdges only once at the end so higher priority
+  // edges are retrieved first, not the ones that happen to be first
+  // in the want_ map.
+  for (std::set<Pool*>::iterator it=pools.begin(),
+           end = pools.end(); it != end; ++it) {
+    (*it)->RetrieveReadyEdges(&ready_);
+  }
+}
+
+void Plan::PrepareQueue() {
+  ComputeCriticalPath();
+  ScheduleInitialEdges();
+}
+
 void Plan::Dump() const {
   printf("pending: %d\n", (int)want_.size());
   for (map<Edge*, Want>::const_iterator e = want_.begin(); e != want_.end(); ++e) {
@@ -606,6 +720,7 @@ bool Builder::AlreadyUpToDate() const {
 
 bool Builder::Build(string* err) {
   assert(!AlreadyUpToDate());
+  plan_.PrepareQueue();
 
   status_->PlanHasTotalEdges(plan_.command_edge_count());
   int pending_commands = 0;

src/build.h

@@ -18,12 +18,11 @@
 #include <cstdio>
 #include <map>
 #include <memory>
-#include <queue>
 #include <string>
 #include <vector>
 
 #include "depfile_parser.h"
-#include "graph.h"  // XXX needed for DependencyScan; should rearrange.
+#include "graph.h"
 #include "exit_status.h"
 #include "util.h"  // int64_t
 
@@ -76,21 +75,13 @@ struct Plan {
   /// Reset state.  Clears want and ready sets.
   void Reset();
 
+  // After all targets have been added, prepares the ready queue for find work.
+  void PrepareQueue();
+
   /// Update the build plan to account for modifications made to the graph
   /// by information loaded from a dyndep file.
   bool DyndepsLoaded(DependencyScan* scan, const Node* node,
                      const DyndepFile& ddf, std::string* err);
-private:
-  bool RefreshDyndepDependents(DependencyScan* scan, const Node* node, std::string* err);
-  void UnmarkDependents(const Node* node, std::set<Node*>* dependents);
-  bool AddSubTarget(const Node* node, const Node* dependent, std::string* err,
-                    std::set<Edge*>* dyndep_walk);
-
-  /// Update plan with knowledge that the given node is up to date.
-  /// If the node is a dyndep binding on any of its dependents, this
-  /// loads dynamic dependencies from the node's path.
-  /// Returns 'false' if loading dyndep info fails and 'true' otherwise.
-  bool NodeFinished(Node* node, std::string* err);
 
   /// Enumerate possible steps we want for an edge.
   enum Want
@@ -105,6 +96,23 @@ struct Plan {
     kWantToFinish
   };
 
+private:
+  void ComputeCriticalPath();
+  bool RefreshDyndepDependents(DependencyScan* scan, const Node* node, std::string* err);
+  void UnmarkDependents(const Node* node, std::set<Node*>* dependents);
+  bool AddSubTarget(const Node* node, const Node* dependent, std::string* err,
+                    std::set<Edge*>* dyndep_walk);
+
+  // Add edges that kWantToStart into the ready queue
+  // Must be called after ComputeCriticalPath and before FindWork
+  void ScheduleInitialEdges();
+
+  /// Update plan with knowledge that the given node is up to date.
+  /// If the node is a dyndep binding on any of its dependents, this
+  /// loads dynamic dependencies from the node's path.
+  /// Returns 'false' if loading dyndep info fails and 'true' otherwise.
+  bool NodeFinished(Node* node, std::string* err);
+
   void EdgeWanted(const Edge* edge);
   bool EdgeMaybeReady(std::map<Edge*, Want>::iterator want_e, std::string* err);
 
@@ -119,9 +127,11 @@ struct Plan {
   /// we want for the edge.
   std::map<Edge*, Want> want_;
 
-  EdgeSet ready_;
+  EdgePriorityQueue ready_;
 
   Builder* builder_;
+  /// user provided targets in build order, earlier one have higher priority
+  std::vector<const Node*> targets_;
 
   /// Total number of edges that have commands (not phony).
   int command_edges_;